Window frame-guided stage mechanism

ABSTRACT

A guided stage mechanism suitable for supporting a reticle in a photolithography machine includes a stage movable in the X-Y directions on a base. Laterally surrounding the stage is a rectangular window frame guide which is driven in the X-axis direction on two fixed guides by means of motor coils on the window frame guide co-operating with magnetic tracks fixed on the base. The stage is driven inside the window frame guide in the Y-axis direction by motor coils located on the stage co-operating with magnetic tracks located on the window frame guide. Forces from the drive motors of both the window frame guide and the stage are transmitted through the center of gravity of the stage, thereby eliminating unwanted moments of inertia. Additionally, reaction forces caused by the drive motors are isolated from the projection lens and the alignment portions of the photolithography machine. This isolation is accomplished by providing a mechanical support for the stage independent of the support for its window frame guide. The window frame guide is a hinged structure capable of a slight yawing (rotational) motion due to hinged flexures which connect the window frame guide members.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to precision motion stages and more specificallyto a stage suitable for use in a photolithography machine and especiallyadapted for supporting a reticle.

2. Description of the Prior Art

Photolithography is a well known field especially as applied tosemiconductor fabrication. In photolithography equipment a stage (an X-Ymotion device) supports the reticle (i.e., mask) and a second stagesupports the semiconductor wafer, i.e. the work piece being processed.Sometimes only a single stage is provided, for the wafer or the mask.

Such stages are essential for precision motion in the X-axis and Y-axisdirections and often some slight motion is provided for adjustments inthe vertical (Z-axis) direction. A reticle stage is typically used wherethe reticle is being scanned in a scanning exposure system, to providesmooth and precise scanning motion in one linear direction and insuringaccurate, reticle to wafer alignment by controlling small displacementmotion perpendicular to the scanning direction and a small amount of"yaw" (rotation) in the X-Y plane. It is desirable that such an X-Ystage be relatively simple and be fabricated from commercially availablecomponents in order to reduce cost, while maintaining the desired amountof accuracy. Additionally, many prior art stages include a guidestructure located directly under the stage itself. This is not adesirable in a reticle stage since it is essential that a light beam bedirected through the reticle and through the stage itself to theunderlying projection lens. Thus a stage is needed which does notinclude any guides directly under the stage itself, since the stageitself must define a fairly large central passage for the light beam.

Additionally, many prior art stages do not drive the stage through itscenter of gravity which undesirably induces a twisting motion in thestage, reducing the frequency response of the stage. Therefore there isa need for an improved stage and especially one suitable for a reticlestage.

SUMMARY

A precision motion stage mechanism includes the stage itself which movesin the X-Y plane on a flat base. The stage is laterally surrounded by a"window frame" guide structure which includes four members attached ator near their corners to form a rectangular structure. The attachmentsare flexures which are a special type of hinge allowing movement topermit slight distortion of the rectangle. In one version these flexuresare thin stainless steel strips attached in an "X" configuration,allowing the desired degree of hinge movement between any two adjacentconnected window frame members.

The window frame guide structure moves on a base against twospaced-apart and parallel fixed guides in e.g. the X axis direction,being driven by motor coils mounted on two opposing members of thewindow frame cooperating with magnetic tracks fixed on the base.

The window frame in effect "follows" the movement of the stage andcarries the magnetic tracks needed for movement of the stage in the Yaxis direction. (It is to be understood that references herein to the Xand Y axes directions are merely illustrative and for purposes oforientation relative to the present drawings and are not to be construedas limiting.)

The stage movement in the direction perpendicular (the Y axis direction)to the direction of movement of the window frame is accomplished by thestage moving along the other two members of the window frame. The stageis driven relative to the window frame by motor coils mounted on thestage and cooperating with magnetic tracks mounted in the two associatedmembers of the window frame.

To minimize friction, the stage is supported on the base by air bearingsor other fluid bearings mounted on the underside of the stage. Similarlyfluid bearings support the window frame members on their fixed guides.Additionally, fluid bearings load the window frame members against thefixed guides and load the stage against the window frame. So as to allowslight yaw movement, these loading bearings are spring mounted. Thestage itself defines a central passage. The reticle rests on a chuckmounted on the stage. Light from an illuminating source typicallylocated above the reticle passes to the central passage through thereticle and chuck to the underlying projection lens.

It is to be understood that the present stage, with suitablemodifications, is not restricted to supporting a reticle but also may beused as a wafer stage and is indeed not limited to photolithographyapplications but is generally suited to precision stages.

An additional aspect in accordance with the present invention is thatthe reaction force of the stage and window frame drive motors is nottransmitted to the support frame of the photolithography apparatusprojection lens but is transmitted independently directly to the earth'ssurface by an independent supporting structure. Thus the reaction forcescaused by movement of the stage do not induce undesirable movement inthe projection lens or other elements of the photolithography machine.

This physically isolating the stage reaction forces from the projectionlens and associated structures prevents these reaction forces fromvibrating the projection lens and associated structures. Thesestructures include the interferometer system used to determine the exactlocation of the stage in the X-Y plane and the wafer stage. Thus thereticle stage mechanism support is spaced apart from and independentlysupported from the other elements of the photolithography machine andextends to the surface of the earth.

Advantageously, the reaction forces from operation of the four motorcoils for moving both the stage and its window frame are transmittedthrough the center of gravity of the stage, thereby desirably reducingunwanted moments of force (i.e., torque). The controller controlling thepower to the four drive motor coils takes into consideration therelative position of the stage and the frame and proportions the drivingforce accordingly by a differential drive technique.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of the present window frame guided stage.

FIG. 2 shows a side view of the window frame guided stage and associatedstructures.

FIGS. 3A and 3B show enlarged views of portions of the structure of FIG.2.

FIG. 4 shows a top view of a photolithography apparatus including thepresent window frame guided stage.

FIG. 5 shows a side view of the photolithography apparatus of FIG. 4.

FIGS. 6A and 6B show a flexure hinge structure as used e.g. in thepresent window frame guided stage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a top view of a stage mechanism in accordance with thepresent invention. See also copending commonly owned and invented U.S.patent application, Ser. No. 08/221,375 entitled "Guideless Stage withIsolated Reaction Stage" filed Apr. 1, 1994, original docket no. NPI0500which is incorporated herein by reference and shows a related method ofsupporting elements of a stage mechanism so as to isolate reactionforces from the projection lens and other parts of a photolithographyapparatus.

The stage 10 is (in plan view) a rectangular structure of a rigidmaterial (e.g., steel, aluminum, or ceramic). Two interferometry mirrors14A and 14B located on stage 10 interact conventionally withrespectively laser beams 16A and 16B. Conventionally, laser beams 16Aare two pairs of laser beams and laser beams 16B are one pair of laserbeam, for three independent distance measurements. The underside ofstage 10 defines a relieved portion 22 (indicated by a dotted line, notbeing visible in the plane of the drawing). A reticle 24 is located onstage 10 and held by conventional reticle vacuum groove 26 formed in theupper surface of chuck plate 28. Stage 10 also defines a centralaperture 30 (passage) below the location of reticle 24. Central aperture30 allows the light (or other) beam which penetrates through reticle 24to enter the underlying projection lens, as described further below. (Itis to be understood that the reticle 24 itself is not a part of thestage mechanism.) Moreover if the present stage mechanism is to be usedfor other than a reticle stage, i.e. for supporting a wafer, aperture 30is not needed.

Stage 10 is supported on a conventional rectangular base structure 32 ofe.g. granite, steel, or aluminum, and having a smooth planar uppersurface. The left and right edges (in FIG. 1) of base structure 32 areshown as dotted lines, being overlain by other structures (as describedbelow) in this view. In operation, stage 10 is not in direct physicalcontact with its base structure 32; instead, stage 10 is verticallysupported by, in this example, conventional bearings such as gasbearings. In one embodiment three air bearings 36A, 36B and 36C are usedwhich may be of a type commercially available.

In an alternative air bearing/vacuum structure, the vacuum portion isphysically separated from and adjacent to the air bearing portion. It isto be understood that the vacuum and compressed air are providedexternally via tubing in a conventional cable bundle and internal tubingdistribution system (not shown in the drawings for simplicity). Inoperation stage 10 thereby floats on the air bearings 36A, 36B, 36Capproximately 1 to 3 micrometers above the flat top surface of basestructure 32. It is to be understood that other types of bearings (e.g.air bearing/magnetic combination type) may be used alternatively.

Stage 10 is laterally surrounded by the "window frame guide" which is afour member rectangular structure. The four members as shown in FIG. 1are (in the drawing) the top member 40A, the bottom member 40B, thelefthand member 40C, and the righthand member 40D. The four members40A-40D are of any material having high specific stiffness (stiffness todensity ratio) such as aluminum or a composite material. These fourmembers 40A-40D are attached together by hinge structures which allownon-rigid movement of the four members relative to one another in theX-Y plane and about the Z-axis as shown in the drawing, this movementalso referred to as a "yaw" movement. The hinge is described in detailbelow, each hinge 44A, 44B, 44C and 44D being e.g. one or more metalflexures allowing a slight flexing of the window frame guide structure.

The window frame guide structure moves in the X axis (to the left andright in FIG. 1) supported on horizontal surfaces of fixed guides 46Aand 46B, and supported on vertical surfaces of fixed guides 64A, 64B.(It is to be understood that each pair of fixed guides 46A, 64A and 46B,64B could be e.g. a single L-shaped fixed guide, or other configurationsof fixed guides may be used.) Mounted on window frame guide member 40Aare two air bearings 50A and 50B that cause the member 40A to ride onits supporting fixed guide member 46A. Similarly air bearings 52A and52B are mounted on the member 40B, allowing member 40B to ride on itssupporting fixed guide member 46B. Air bearings 50A, 50B, 52A, 52B aresimilar to air bearings 36A, etc.

The window frame guide is driven along the X axis on fixed guides 46Aand 46B, 64A and 64B by a conventional linear motor, which includes acoil 60A which is mounted on window frame guide member 40A. Motor coil60A moves in a magnetic track 62A which is located in (or along) fixedguide 64A. Similarly, motor coil 60B which is mounted on window frameguide member 40B moves in magnetic track 62B which is located in fixedguide 64B. The motor coil and track combinations are part no. LM-310from Trilogy Company of Webster Tex. These motors are also called"linear commutator motors". The tracks 62A, 62B are each a number ofpermanent magnets fastened together. The electric wires which connect tothe motor coils are not shown but are conventional. Other types oflinear motors may be substituted. It is to be understood that thelocations of the motor coils and magnetic tracks for each motor could bereversed, so that for instance the magnetic tracks are located on stage10 and the corresponding motor coils on the window frame guide members,at a penalty of reduced performance.

Similarly, stage 10 moves along the Y axis in FIG. 1 by means of motorcoils 68A and 68B mounted respectively on the left and right edges ofstage 10. Motor coil 68A moves in magnetic track 70A mounted in windowframe guide member 40C. Motor coil 68B moves in magnetic track 70Bmounted in window frame guide member 40D.

Also shown in FIG. 1 are air bearings 72A, 72B and 72C. Air bearing 72Ais located on window frame guide member 40A and minimizes frictionbetween window frame guide member 40A and its fixed guide 64A. Similarlytwo air bearings 72B and 72C on window frame guide member 40B minimizeits friction with the fixed guide 64B. The use of a single air bearing72A at one end and two opposing air bearings 72B and 72C at the otherend allows a certain amount of yaw (rotation in the X-Y plane about theZ-axis) as well as limited motion along the Z-axis. In this case,typically air bearing 72A is gimbal mounted, or gimbal mounted with thegimbal located on a flexure so as to allow a limited amount ofmisalignment between the member 40A and fixed guide 64A.

The use of the air bearing 72A opposing bearings 72B and 72C provides aloading effect to keep the window frame guide in its proper relationshipto fixed guides 64A, 64B. Similarly, an air bearing 76A loads opposingair bearings 76B and 76C, all mounted on side surfaces of the stage 10,in maintaining the proper location of stage 10 relative to the opposingwindow frame guide members 40B and 40D. Again, in this case one airbearing such as 76A is gimbal mounted to provide a limited amount ofmisalignment, or gimbal mounted with the gimbal on a flexure (spring).Air bearings 72A, 72B, 72C and 76A, 76B, and 76C are conventional airbearings.

The outer structure 80 in FIG. 1 is the base support structure for thefixed guides 46A, 46B, 64A, 64B and the window frame guide members 40A,. . . , 40D of the stage mechanism, but does not support stage basestructure 32. Thus the underlying support is partitioned so the reactionforce on base support structure 80 does not couple into the stage basestructure 32. Base support structure 80 is supported by its own supportpillars or other conventional support elements (not shown in thisdrawing) to the ground, i.e. the surface of the earth or the floor of abuilding. An example of a suitable support structure is disclosed inabove-referenced U.S. patent application Ser. No. 08/221,375 at FIGS. 1,1B, 1C. This independent support structure for this portion of stagemechanism provides the above-described advantage of transmitting thereaction forces of the reticle stage mechanism drive motors away fromthe frame supporting the other elements of the photolithographyapparatus, especially away from the optical elements including theprojection lens and from the wafer stage, thereby minimizing vibrationforces on the projection lens due to reticle stage movement. This isfurther described below.

The drive forces for the stage mechanism are provided as close aspossible through the stage mechanism center of gravity. As can beunderstood, the center of gravity of the stage mechanism moves with thestage 10. Thus the stage 10 and the window frame guide combine to definea joint center of gravity. A first differential drive control (notshown) for motor coils 60A, 60B takes into account the location of thewindow frame guide to control the force exerted by each motor coil 60A,60B to keep the effective force applied at the center of gravity. Asecond conventional differential drive control (not shown) for motorcoils 68A, 68B takes into account the location of stage 10 to controlthe force exerted by each motor coil 68A, 68B to keep the effectiveforce applied at the center of gravity. It is to be understood thatsince stage 10 has a substantial range of movement, that thedifferential drive for the motor coils 60A, 60B has a wide differentialswing. In contrast, the window frame guide has no CG change, hence thedifferential drive for the motor coils 68A, 68B has a much lesserdifferential swing, providing a trim effect. Advantageously, use of thewindow frame guide maintains the reaction forces generated by movementof the reticle stage mechanism in a single plane, thus making easier toisolate these forces from other parts of the photolithography apparatus.

FIG. 2 shows a cross-sectional view through line 2--2 of FIG. 1. Thestructures shown in FIG. 2 which are also in FIG. 1 have identicalreference numbers and are not described herein. Also shown in FIG. 2 isthe illuminator 90 which is a conventional element shown here withoutdetail, and omitted from FIG. 1 for clarity. Also shown without detailin FIG. 2 is the upper portion of the projection lens (barrel) 92. It isto be understood that the lower portion of the projection lens and otherelements of the photolithography apparatus are not shown in FIG. 2, butare illustrated and described below.

The supporting structure 94 for the projection lens 92 is also shown inFIG. 2. As can be seen, structure 94 is separated at all points by aslight gap 96 from the base support structure 80 for the reticle stagemechanism. This gap 96 isolates vibrations caused by movement of thereticle stage mechanism from the projection lens 92 and its support 94.As shown in FIG. 2, stage 10 is not in this embodiment a flat structurebut defines the underside relieved portion 22 to accommodate the upperportion of lens 92. Magnetic track 70A is mounted on top of the windowframe guide 40B and similarly magnetic track 70B is mounted on top ofthe opposite window frame guide member 40D.

FIGS. 3A and 3B are enlarged views of portions of FIG. 2, with identicalreference numbers; FIG. 3A is the left side of FIG. 2 and FIG. 3B is theright side of FIG. 2. Shown in FIG. 3A is the spring mounting 78 for airbearing 76A. Air bearing 78A being spring mounted to a side surface ofstage 10, this allows a certain amount of yaw (rotation in the X-Y planeabout the Z-axis) as well as limited motion along the Z-axis. A gimbalmounting may be used in place of or in addition to the spring 78. Thespring or gimbal mounting thereby allows for a limited amount ofmisalignment between stage 10 and members 40C, 40D (not shown in FIG.3A).

FIG. 4 is a top view of a photolithography apparatus including the stagemechanism of FIGS. 1 and 2 and further including, in addition to theelements shown in FIG. 1, the supporting base structure 100 whichsupports the photolithography apparatus including frame 94 except forthe reticle stage mechanism. (Not all the structures shown in FIG. 1 arelabelled in FIG. 4, for simplicity.) Base structure 100 supports fourvertical support pillars 102A, 102B, 102C and 102D connected tostructure 94 by respectively bracket structures 106A, 106B, 106C and106D. It is to be appreciated that the size of the base structure 100 isfairly large, i.e. approximately 3 meters top to bottom in oneembodiment. Each pillar 102A, 102B, 102C, 102D includes an internalconventional servo mechanism (not shown) for leveling purposes. Alsoshown in FIG. 4 are the supports 108 and 110 for respectively laserinterferometer units (beam splitter etc.) 112A, 112B, 112C. FIG. 4 willbe further understood with reference to FIG. 5 which shows a view ofFIG. 4 through cross-sectional line 5--5 of FIG. 4.

In FIGS. 4 and 5 the full extent of the supporting structure 94 can beseen along with its support pillars 102A, 102C which rest on the basestructure 100 which is in contact with the ground via a conventionalfoundation (not shown). The independent support structure for thereticle stage base support structure 80 is shown, in FIG. 4 only (forclarity) and similarly includes a set of four pillars 114A, 114B, 114C,114D with associated bracket structures 116A, 116B, 116C, 116D, with thepillars thereby extending from the level of base support structure 80down to the base structure 100.

The lower portion of FIG. 5 shows the wafer stage 120 and associatedsupport structures 122, 124. The elements of wafer stage 120conventionally include (not labelled in the drawing) a base, the stageitself, fixed stage guides located on the base, magnetic tracks locatedon the fixed stage guides, and motor coils fitting in the magnetictracks and connected to the stage itself. Laser beams from laser 124mounted on support 126 locate lens 92 and the stage itself byinterferometry.

FIG. 6A shows detail of one of the window frame guide hinged flexurestructures, e.g. 44C, in a top view (corresponding to FIG. 1). Each ofhinges 44A, 44B, 44C and 44D is identical. These flexure hinges have theadvantage over a mechanical-type hinge of not needing lubrication, notexhibiting histeresis (as long as the flexure is not bent beyond itsmechanical tolerance) and not having any mechanical "slop", as well asbeing inexpensive to fabricate.

Each individual flexure is e.g. 1/4 hard 302 stainless steelapproximately 20 mils (0.02 inch) thick and can sustain a maximum bendof 0.5 degree. The width of each flexure is not critical; a typicalwidth is 0.5 inch. Two, three or four flexures are used at each hinge44A, 44B, 44C and 44D in FIG. 1. The number of flexures used at eachhinge is essentially determined by the amount of space available, i.e.,the height of the window frame guide members. The four individualflexures 130A, 130B, 130C, 130D shown in FIG. 6A (and also in a 90°rotated view in FIG. 6B) are each attached by clamps 136A, 136B, 136C,136D to adjacent frame members (members 40B and 40D in FIGS. 6A and 6B)by conventional screws which pass through holes in the individualflexures 130A, 130B, 130C, 130D and through the clamps and are securedin corresponding threaded holes in frame members 40B and 40D.

Note that the frame members 40B, 40D of FIGS. 6A and 6B differ somewhatfrom those of FIG. 1 in terms of the angular (triangular) structures atthe ends of frame members 40B, 40D and to which the metal flexures 130A,130B, 130C, 130D are mounted. In the embodiment of FIG. 1, these angularstructures are dispensed with, although their presence makes screwmounting of the flexures easier.

In an alternate embodiment, the window frame guide is not hinged but isa rigid structure. To accommodate this rigidity and prevent binding, oneof bearings 72C or 72B is eliminated, and the remaining bearing moved tothe center of member 40B, mounted on a gimbal with no spring. The otherbearings (except those mounted on stage 10) are also gimballed.

This disclosure is illustrative and not limiting; further modificationswill be apparent to one skilled in the art in light of this disclosureand are intended to fall within the scope of the appended claims.

I claim:
 1. A stage mechanism moveable in two dimensions, comprising:abase defining a principal surface; a movable stage located on theprincipal surface of the base; a frame of four members flexiblyconnected together and laterally surrounding the stage therebypermitting a yaw movement of the frame in a plane defined by theprincipal surface of the base, a first and second of the members beingopposing and in slidable contact with the stage; and first and secondfixed guides mounted spaced apart and parallel to each other, a thirdand fourth of the members being opposing and in slidable contact withrespectively the first and second fixed guides.
 2. The stage mechanismof claim 1, wherein the first and second members are each connected toboth the third and fourth members by flexures, thereby allowing movementof each member relative to the other members.
 3. The stage mechanism ofclaim 2, wherein each flexure comprises at least two strips of metal,the two strips each being connected to two of the members, and the twostrips overlying one another in an X configuration.
 4. The stagemechanism of claim 1, further comprising a plurality of fluid bearingslocated on an underside of the stage and confronting the base.
 5. Thestage mechanism of claim 1, further comprising at least two fluidbearings located on each of the third and fourth members andconfronting, respectively, surfaces of the first and second fixedguides.
 6. The stage mechanism of claim 1, further comprising a firstmotor element on each of the third and fourth members, thereby drivingthe frame relative to the first and second fixed guides.
 7. The stagemechanism of claim 6, further comprising a second motor element on thebase and associated with each of the respectively first and second fixedguides co-operating with one of the first motor elements.
 8. The stagemechanism of claim 1, further comprising a first motor element locatedon opposing sides of the stage confronting respectively the first andsecond members, thereby driving the stage relative to the first andsecond members.
 9. The stage mechanism of claim 1, further comprising afluid bearing spring mounted on one of the third and fourth members andconfronting a surface of one of the first and second fixed guides,thereby loading the frame against the first and second fixed guides. 10.The stage mechanism of claim 1, further comprising a support base forthe first and second fixed guides and the frame members, the supportbase being supported independently of the base on which the stage islocated.
 11. The stage mechanism of claim 1, further comprising a fluidbearing gimbal mounted on one of the third and fourth members andconfronting a surface of one of the first and second fixed guides,thereby loading the frame against the first and second fixed guides. 12.A method of operating a stage moveable in two perpendicular directions,comprising the steps of:locating the stage on a base, the stage beingadapted for flexing in the two perpendicular directions; laterallysurrounding the stage with a frame in slidable contact with the stage;driving the frame in a first of the two perpendicular directionsrelative to the base; and driving the stage in a second of the twoperpendicular directions relative to the frame.
 13. The method of claim12, further comprising supporting the base independently from the frame.14. A stage mechanism moveable in two dimensions, comprising:a basedefining a principal surface; a movable stage located on the principalsurface of the base; a frame of four members connected together andlaterally surrounding the stage, a first and second of the members beingopposing and in slidable contact with the stage, wherein the first andsecond members are each connected to both the third and fourth membersby flexures, thereby allowing movement of each member relative to theother members; and first and second fixed guides mounted spaced apartand parallel to each other, a third and fourth of the members beingopposing and in slidable contact with respectively the first and secondfixed guides.
 15. A stage mechanism moveable in two dimensions,comprising:a base defining a principal surface; a movable stage locatedon the principal surface of the base; a frame of four members connectedtogether and laterally surrounding the stage, a first and second of themembers being opposing and in slidable contact with the stage; first andsecond fixed guides mounted spaced apart and parallel to each other, athird and fourth of the members being opposing and in slidable contactwith respectively the first and second fixed guides; a first linearmotor disposed between the third member and the first fixed guide; asecond linear motor disposed between the fourth member and the secondfixed guide; and a differential drive control connected to the first andsecond linear motors.